This present invention generally relates to tactile input devices and more specifically to tactile input devices with integral touch input surfaces and force-measuring capability.
Tactile input devices, that quantify the level of force being applied to a surface, are becoming more popular in a variety of applications. Cellular telephones, smartphones, tablets, laptops, personal digital assistants (PDAS), “smart” watches and other electronic devices seek to incorporate greater numbers of functions in such devices for a given volume. Tactile input devices can provide this capability. This has led to a number of approaches for providing input devices that utilize capacitive, resistive, strain gauge and other types of sensors.
For example, U.S. Pat. No. 7,609,178 to Son et al. discloses a reconfigurable tactile sensor input device. The input device uses capacitive sensing and includes a first rigid electrode layer, a compressible dielectric structure and a second flexible electrode layer. The dielectric structure may include a matrix of compressible geometric elements with voids therebetween. This approach provides the device with the ability to measure varying levels of tactile force. An optional flexible display may be mounted over the input device to indicate the present configuration to the device, which configuration can change from time to time based upon various inputs.
U.S. Pat. No. 7,595,788 to Son discloses an electronic device housing with an integrated user input capability. This device measures the force level applied by the individual to a housing for an electronic device. A capacitive sensor is formed between a conductive inside surface of the housing and an electrode on a rigid base. The application of force to the housing deflects a corresponding portion thereby causing a change in capacitance across the sensor.
In addition to the developments such as those described above, efforts have also been directed to the development of touch location devices, such as touch screens. U.S. Pat. No. 7,148,882 to Kamrath et al. discloses a complex to build capacitor-based force sensors that detect a difference caused by an applied force over a continuous range of applied forces that includes a zero force. A number of these sensors are distributed under an input structure. When a force is applied, the forces as measured by the capacitance properties of each sensor are converted into the position of the applied force on the input structure.
U.S. Pat. No. 7,183,948 to Roberts discloses a tangential force control touch location device that includes force sensor principal elements, lateral softening means and preload springs. A mechanical path, that does not include the force sensors, comprises a plurality of shunt connections that constitute the lateral stiffening means for impeding lateral motion of the touch surface.
Each of the foregoing references discloses different approaches. Manufacturers who wish to incorporate a touch screen with a tactile input device such as shown in the above-identified patent publications purchase the input device from one vendor and the touch screen device from another vendor. Then the manufacturer must determine an approach for integrating the two. Generally the electronics associated with each component are not compatible, so different processing approaches must be undertaken. This may further increase the price and size of the input device to accommodate the different circuits in the device or require more complex signal transfers to external electronic and processing components.
In general, there are several methods to measure force on a touch-sensitive screen. This can be done by:                using sensors on the corners of the display (such as in a Force Pad),        deploying pressure sensors surrounding the display (as in Apple Watch™)        using a deformation of the cover plate on top of a capacitive sensing display (as in Apple 6S iPhone™), or        positioning pressure sensors beneath the display.        
Several examples of prior art disclose positioning one or more thin pressure sensors underneath the display. Display is then allowed to slightly flex elastically or move down and compress spring-loaded supporting structure when an external force is applied to the screen surface. Force sensors may be adapted to capture that motion of the display and transform into a force signal. U.S. Pat. No. 8,169,332 to Son discloses one such design. Another example may be found in Japanese Pat App. No. JP 2011-100364.
Straight forward deployment of an array of thin film pressure sensors underneath the display presents a disadvantage of being too expensive because of multiple layers of electrodes needed to fabricate the sensor.
Using such devices in cellular phones, tablets or other electronics dictates a requirement of extra reliability as these devices may be occasionally dropped, mishandled, compressed or subjected to other conditions of shock and vibration.
The need exists therefore for a force-sensing input device that provides a touch screen display along with optional various keyboard configurations whereby the cost of the device is minimized and reliability of assembly is enhanced so as to make it practical for use in cellular phones, tablets and other small or thin electronics.